In the field of arc welding, the main types of welding processes are gas-metal arc welding with solid (GMAW) or metal-cored wires (GMAW-C), gas shielded flux-cored arc welding (FCAW-G), self shielded flux-cored arc welding (FCAW-S), shielded metal arc welding (SMAW) and submerged arc welding (SAW). Of these processes, gas metal arc welding with solid or metal-cored electrodes are increasingly being used for joining or overlaying metallic components. These types of welding processes are becoming increasingly popular because such processes provide increased productivity and versatility. Such increase in productivity and versatility results from the continuous nature of the welding electrodes in gas metal arc welding (GMAW & GMAW-C) which offers substantial productivity gains over shielded metal arc welding (SMAW). Moreover, these electrodes produce very good looking welds with very little slag, thus saving time and expense associated with cleaning welds and disposing of slag, a problem that is often encountered in the other welding processes.
In gas metal arc welding with solid or cored electrodes, a shielding gas is used to provide protection for the weld against atmospheric contamination during welding. Solid electrodes are appropriately alloyed with ingredients that, in combination with the shielding gas, provide porosity free welds with the desired physical and mechanical properties. In cored electrodes, these ingredients are on the inside, in the core (fill) of a metallic outer sheath, and provide a similar function as in the case of solid electrodes.
Solid and cored electrodes are designed to provide, under appropriate gas shielding, a solid, substantially porosity free weld with yield strength, tensile strength, ductility and impact strength to perform satisfactorily in the final applications. These electrodes are also designed to minimize the quantity of slag generated during welding. Cored electrodes are used increasingly as an alternative to solid wires because of increased productivity during welding fabrication of structural components. Cored electrodes are composite electrodes consisting of a core (fill) material surrounded by a metallic outer sheath. The core consists mainly of metal powder and fluxing ingredients to help with arc stability, weld wetting and appearance etc., such that the desired physical and mechanical properties are obtained in the weld. Cored electrodes are manufactured by mixing up the ingredients of the core material and depositing them inside a formed strip, and then closing and drawing the strip to the final diameter. Cored electrodes provide increased deposition rates and produce a wider, more consistent weld penetration profile compared to solid electrodes. Moreover, they provide improved arc action, generate less fume and spatter, and provide weld deposits with better wetting compared to solid electrodes.
In submerged arc welding, coalescence is produced by heating with an electric arc between a bare-metal electrode and the metal being worked. The welding is blanketed with a granular or fusible material or flux. The welding operation is started by striking an arc beneath the flux to produce heat to melt the surrounding flux so that it forms a subsurface conductive pool which is kept fluid by the continuous flow of current. The end of the electrode and the work piece directly below it become molten and molten filler metal is deposited from the electrode onto the work. The molten filler metal displaces flux pool and forms the weld. In shielded metal arc welding, shielding is obtained by a flux coating instead of a loose granular blanket of flux.
In the art of welding, much prior effort has been expended in developing flux compositions of the type having predetermined flux components intended to perform in predetermined manners. A large number of compositions have been developed for use as fluxes in arc welding. Fluxes are utilized in arc welding to control the arc stability, modify the weld metal composition, and provide protection from atmospheric contamination. Arc stability is commonly controlled by modifying the composition of the flux. It is therefore desirable to have substances which function well as plasma charge carriers in the flux mixture. Fluxes also modify the weld metal composition by rendering impurities in the metal more easily fusible and providing substances with which these impurities may combine, in preference to the metal to form slag. Other materials may be added to lower the slag melting point, to improve slag fluidity, and to serve as binders for the flux particles.
Cored electrodes are commonly used in electric arc welding of steel base metals. These electrodes generally yield high strength welds in a single pass and multiple passes at high welding speeds. These electrodes are formulated to provide a solid, substantially nonporous weld bead with tensile strength, ductility and impact strength to meet the desired end use of various applications.
One of the many challenges during the formation of a weld metal is to reduce the amount of diffusible hydrogen in the weld bead. Diffusible hydrogen is a known cause of cracking in weld beads. Many studies have shown that an increased amount of moisture content in the flux system results in an increased amount of diffusible hydrogen in the weld metal. During welding, the heat evaporates and dissociates the water, evolving hydrogen gas, which can dissolve into the metal. Hydrogen in the weld metal can result in hydrogen induced cracking and eventual detrimental failure of the weld. Hydrogen embrittlement is a phenomenon which involves loss of ductility and increased crack susceptibility in steel at room temperature due to the presence of hydrogen in the steel. Hydrogen induced cracking can occur to some extent whenever sufficient hydrogen and stress are present in a hard steel at temperatures above −100° C. and below 150° C. Sodium and potassium silicate are commonly used as arc stabilizers and sometimes used in binder systems for flux components. Potassium silicate is known for its high moisture pick-up tendencies.
Another challenge during the formation of a weld metal is to control the amount and effect of impurities in the weld metal. Many of the flux components are derived from natural sources, thus have impurities contained within such components. One common flux component is titanium dioxide (TiO2). This component is commonly added to a flux system in the form of rutile. There are many different sources of rutile throughout the world. Each one of these rutile sources includes different amounts and types of impurities. In flux systems wherein rutile comprises a significant portion of the flux system, these impurities can adversely affect the resulting weld metal. For instance, many forms of rutile include small amounts of niobium and/or vanadium. These two components in small quantities can cause carbide formation in the weld metal, thereby resulting in increased brittleness of the weld metal. Carbide formation can also result in high stress to the weld metal which can lead to cracking of the welding metal and a reduction in the impact toughness of the weld metal. Carbide formation in the weld metal is especially detrimental in multi-pass welding procedures.
In view of the present state of the art flux systems, there is a need for flux system having a reduced moisture content and a reduced amount of impurities so as to form a higher quality weld bead.